Electroluminescence devices



March 22, 19 0 Filed Dec. 28, 1956 J, M. N. HANLET ELECTROLUMINESCENCEDEVICES 2 Sheets-Sheet l FIGA Wu/ZMW mwam March 22, 1960 Filed Dec. 28,1956 J. M. N. HANLET ELECTROLUMINESCENCE DEVICES 2 Sheets-Sheet 2ELECTROLUMINESCENCE DEVICES Jacques Marie Noel Harriet, Paris, France,assignor to- Societe dElectronique et dAutomatisme, Courbevoie, FranceApplication December 28, 1956, Serial No. 631,223

Claims priority, application France December Sit, 1955 11 Claims. (Cl.313-108) The present invention relates to electroluminescent devices ofthe kind wherein a thin semi-dielectric layer of electroluminescentmaterial is inserted between a pair of electrodes one of which at leastis translucent and wherein the electroluminescence is obtained by theapplication of an A.C. voltage across the said electrodes.

The object of the invention is to provide improved structures of devicesof the above kind so that their electro-optical eiiiciency issubstantially increased, at least in a ratio from five to one withrespect to the ele mentary structure of these devices.

According to the invention, this increase of efiiciency is obtained byincorporating in such an electroluminescent unit a photomultiplier unitconstituted by a semiconductive layer and a further electroluminescentlayer, themselves inserted between a pair of translucent electrodes oneof which is integral with the electrode of the electroluminescent unitwhich is translucent and applying an A.C. voltage at least across theelectrodes of the electrolurninescent cell and a D.C. voltage across theelectrodes of the photomultiplier cell.

According to a further feature of the invention, the saidphotomultiplier unit includes between the semi-conductive layer and theelectroluminescent layer thereof, a further layer of a highly conductivematerial, which is preferably opaque to the light emitted from the saidelectroluminescent layer, for cancelling therein any potential barrierbetween the said semi-conductive and electroluminescent layers and,preferably also, avoiding any retroaction of the light issued from thesaid electroiuminescent layer upon the said semi-conductive layer.

It is obvious that there may be provided successive photomultiplierunits in the device, each one having an electrode common with the nextpreceding one, in the direction of transmission and amplification oflight therethrough.

In the accompanying drawings, Fig. 1 shows a plain constitution of anelectroluminescent device according to the invention whereas Fig. 2discloses a more elaborate structure thereof.

Referring first to Fig. 1, an electrode 1 is coated with a thin layer ofa semi-dielectric electroluminescent material 2 and, on the other faceof the said layer, is applied another conducting electrode 3, such as afilm of evaporated silver or gold, or the like. The electrode 1 may beopaque, if required and, in such a case, will preferably be polished tothe degree used in optical surfaces for a good reflection of lighttherefrom. These three elements define together the electroluminescentunit proper of the device. The activation or drive thereof is made byapplying between the armatures 1 and 3 a suitable A.C. electromotiveforce, B

A photomultiplier unit is combined with such an electroluminescent unitby virtue of the electrode 3 also constitutes the first electrode of thesaid photomultiplier unit. The latter includes: a semi-conducting layer4 upon which is coated a second electroluminescent layer 5, of t e. s mee a ha acter as th semidielectril;

layer 2, and a third translucent electrode 6 is coated over the layer 5.Across the electrodes 3 and 6 of the said photomultiplier unit isapplied a DC. electro-motive force, E

The connections of these e.m.f. sources must be such that no directcurrent can flow up to the armature 1 of the device, through the A.C.source. This connection may be made through a decoupling condenser, asis usual for this kind of coupling between an A.C. and a DC. circuit.

The operation may be stated as follows: The A.C.

source produces the activation of electroluminescence within the layer 2and the light emitted therefrom passes through the translucent electrode3 to the semi-conductor 4 of the photomultiplier unit. Thesemi-conductor 4 is chosen to be photoelectric with respect to the lightfrom the said layer 2. The photomultiplier unit is unable to produce alight emission from the layer 5 when it is driven solely by the DC.voltage E However this layer 5 will be activated, when in the presenceof the said DC. bias, the light from the electroluminescent layer 2varies in accordance with the A.C. drive thereof. The semi-conductinglayer 4 is energized by the said fluctuation and the resistivity thereofvaries accordingly, and consequently varies the voltage difierenceacross the electroluminescent layer 5. The said layer is then acetivated and emits the light from its own luminescence through theconducting and translucent electrode 6. It is the properties of thesemi-conducting layer 4 which are preponderant in the light intensifyingeffect of the device. Even if some light passes through the said layer 4and falls upon the semi-dielectric layer 5, this action is quiteunimportant with respect to the overall efliciency. Theelectroluminescent layer 5 is submitted to a potential difierence equalto the DC. potential value E minus the drop within the semi-conductinglayer 4, and which con-.

stitutes the variable part of the activating voltage for this layer 5.It is quite apparent that the average energy level within this layer 5will be appreciably greater than the corresponding level in the layer 2.Consequently the layers 2 and 5 may be made of the same material and theresult of an increased overall efiiciency be obtained in such a simplecase.

in a preferred embodiment of a device according to the invention, thesemi-conducting layer 4 is made of a monocrystalline body of cadmium,lead or antimony sulphide. A polycrystalline body of selenium may alsobe used but this latter is less efiicient than the former composition ofthe layer 4. The semi-dielectric electroluminescent layers 2 and 5 mayeach be made of a composition containing zinc or cadmium or barium orcalcium sulphide with impurities of metallic salts for the activationthereof. Preferably however, use will be made of the special all-oxidecompositions described in my copending application Serial No. 631,226,filed Dec. 28, 1956. Such compositions comprise more than 99% of a Zinc,barium, calcium or cadmium oxide and the complement of a copper, lead,selenium, thallium, germanium or manganese oxide. 1

Further to the advantage of ultilising the highly efficient propertiesof such compositions, their use also presents a special advantage forthe manufacturing of the device proper. This may be termed as follows:Within a vessel the atmosphere of which may be easily controlled, isfirst formed upon an auxiliary base plate of magnesium or aluminium alayer of an alloy of the, metals or metalloids of the unoxidizedcomponent materials of the electroluminescent layer 2 to be made. Forinstance, this operation is made by selectively evaporating copper andzinc, in suitable proportions as above said, under a vacuum of say 10-millimetres of Hg within the said vessel. 7 The raw materials are placedinto cr lc ia raamrea run. 22, 1960.

alloy onto the electrode which for instance is made of rhodium and hasan optical polish. In the said transfer process the alloy particles areoxidized and, further, the

electrode is heated for instance from a high frequency heater to atemperature suitable for the progressive crystallization of the oxidecomposition depositing upon the said electrode plate 1. A monocrystal ofthe said composition is thus obtained.

This method of making a monocrystalline electroluminescent layer isfully described in my copending application Serial No. 631,224, filedDecember 28, 1956.

Upon the layer 2 a translucent electrode 3 is then deposited, forinstance within the said vessel which is, for that part of the process,evacuated for the evaporation of a film of aluminium, for instance,which will constitutethe said electrode 3.

The above process is repeated by introducing a base plate of magnesiumbearing a layer of cadmium. This layer has for instance been obtainedfrom a similar procass of selective evaporation of a raw material. Thetransfer of the cadmium is made from the same ionic discharge process asabove described, but in an atmosphere of. H 8 at a pressure of about 10-mm. of Hg. The D.C. voltage is of about 600 volts between the cadmiumand the electrode 3 for a spacing thercbetween equal to 9.5 millilitres,and the ionic discharge current is of about 100 milliamperes per squaredecimetre. The cadmium particles extracted from the cadmium layer areconverted into particles of cadmium sulphide and applied to theelectrode 3 as a uniform coating thereto. A similar high frequencyheating of the electrode plate ensures the crystallisation of the saidcadmium sulphide layer so that finally a monocrystal of cadmium sulphideis formed upon the electrode 3 of the device. The temperature ofcrystallisation of the cadmium sulphide is of about 312 C. in the abovesaid conditions. The thickness of the cadmium sulphide monocrystal maybe about microns whereas the thickness of the layer 2 was about 10microns.

The cadmium bearing auxiliary plate is removed and the oxygen atmosphereis re-established after the reintroduction into the vessel of theauxiliary plate which bears the zinc-copper alloy. The operation isrepeated for the formation upon the cadmium sulphide layer of a secondlayer of electroluminescent oxides, as a monocrystalline coating 5, of10 microns of thickness.

Finally the electrode 6 is formed in the same manner as the electrode 3.The device is now completed. i In certain applications of the deviceaccording to the invention, it may prove useful to constitute theelectrodes thereof as networks of conducting lines and usually thedirections of the said conducting lines will vary from one electrode tothe next one in the device. For instance the electrodes 1 and 6 willhave their conducting lines parallel and the conducting lines of thequency which is a multiple ofthe frequency of the first is applied tothe conductors of the electrode 3. The electrode 6 may be lift undividedor else controlled together with the electrode tin such a scanningprocess. A modulating signal may be superimposed on the scanningvoltages for the reproduction of the image repre' sented by the saidmodulating signal.

Fig. 2 shows an arrangement of the device of Fig. 1 adapted for such ause, and also including further improvements with respect to theembodiment shown in Fig. l. The embodiment of Fig. 2 includes a layer 7inserted between the semiconducting layer 4 and the electroluminescentlayer a of the photomultiplier part of the device.

The purpose of this layer 7 is two-fold and actually two distinct layersmight have been provided for two special purposes but, when according toa feature of the inventionfi the material of the layer 7 is chosen as asuitable metal or metallic salt, a single element may be used. Thematerial of the layer 7 may be either indium or, preferably, indiumantimonide. In such arrangements as that of Fig. 1, it will be apparentthat some reduction of efiiciency could arise from the fact that anelectrical potential barrier eifect is created between thesemiconducting layer and the semi-dielectric layer of thephotomultiplier part of the device. The value of the D.C. potentialdifference E must then be higher than if this elfect were negligible,and consequently the e'fficiency of the device may be increased bycancelling or at least reducing to a negligible value the barriereffect. The insertion of a layer 7 such as above said produces thisreduction. But, in the arrangement of Fig. 2,-further, this layer 7-is,made of small separate spots, as obtained from a deposition of indiumantimonide through a meshed screen, the meshes of which have beenprovided in registration with the points defined by the conducting linesof the electrodes 1 and 3. Of course, when the electrodes are madeuniform, the layer 7 may also be made uniform.

When at least the points to become electroluminescent are predeterminedby a form of the electrodes of the device, it will be of specialadvantage that the defini-. tion" of these points cannot be destroyedfrom the back action of the spots which emit light in the layer 5. Theprovision of a suitably opaque layer between the photoelectricsemi-conducting layer 4 and the said layer 5 will cancel such an action.As, however, the material used for layer 7 may be opaque to the lightemitted from the layer 5, it will not be useful to provide a speciallayer and, with the layer 7, of indium antimonide, the two effects,electrical and optical, are achieved withthe provision of a singleadditional layer instead of two.

I claim:

1. An electroluminescent device comprising the combination of anelectroluminescent cell and a photomultiplier cell each having an endelectrode and a common translucent electrode, said photomultiplier cellincluding at least one photoconductive layer over the said electrode andone electroluminescent layer .over said photocopductive layer, a sourceof alternating current connected across the electrodes of the saidelectroluminescent cell and a source of direct current connected acrossthe electrodes of the said photomultiplier cell.

2. .An'electroluminescent device according to claim .1,

wherein the 'said alternating current is also applied across the endelectrodes of the device.

3. An electroluminescent device according to claim 1,, wherein anelectrically highly conductive layer is interposed between the saidphotoconductive and electroluminescent layers of the photomultipliercell.

4. An electroluminescent device according to claim 3, wherein the saidconductive layer is opaque to the'li'ght emitted from theelectroluminescent layer.

'5. An electroluminescent device according to claim 4, wherein thematerial of the said conductive and opaque layer is chosen from a groupconsisting of indium and indium antimonide.

6. An electroluminescent device according to claim 1, wherein each ofthe electrodes comprises a plurality of separate conductors, in the formof a grid, the conductors of adjacent electrodes extending in diflerentdirections, the conductors of the common electrode extending in adiiierent direction with respect to the end electrodes of the device.

7. An electroluminescent device according to claim 6, wherein thealternating current is applied between one end electrode and theremaining electrodes in parallel.

8. An electroluminescent device according to claim 6, wherein spots of ahigh conductivity material are interposed between the saidphotoconductive and electroluminescent layers of the photomultipliercell at the spatial intersections of the conductors of the electrodegrids of the photomultiplier cell.

9. An electroluminescent device according to claim 8,

wherein the material of the said spots is opaque to the light emittedfrom the electroluminescent layer of the photomultiplier cell.

10. An electroluminescent device according to claim 9, wherein the saidmaterial is chosen from a group consisting of indium and indiumantimonide.

11. An electroluminescent device according to claim 1, wherein the endelectrode of the electroluminescent cell is light reflecting.

References Cited in the file of this patent UNITED STATES PATENTS2,698,915 Piper Ian. 4, 1955 2,730,644 Michlin Jan. 10, 1956 2,735,049De Forest Feb. 14, 1956' 2,748,304 'Botden May 29, 1956 2,792,447 KazanMay 14, 1957 2,798,823 Harper July 9, 1957

